Why lucid dreamers always tend to go through doors, walls and windows ? I knew somebody who can’t go through his room door, and as his LD frequently started in his room, he wasted a lot of time flying in it.
Do some lucid dreamers here ever think they also can open the door or the window ? 
Umm, i have went through doors and flyed out of window several times in my LD-s. But as i recall them they were always open (or someone opened them before me) anyway, so no need to go through them, or open them
But i might try sometimes to go THROUGH them to see can i, or not 
watch out, if you go though a stained glass window your dream gets fucked up in all the colors
Opening doors and windows is the easy solution. But in dreams you have the power to practice your mind to become more flexible. How to do that? For instance by doing things which can never be done irl. In order to succeed in doing it in the dream, you need to learn how to put aside all your expectations and associations with real life. Only then, you can for instance walk through walls, doors and windows. It’s just a matter of training the mind to become more flexible and free. At least in my case 
Seems so. I think that flexibility is the key for everything “impossible” in our dreams, so thanks for reminding that. In my last LD i did not succeed when trying to fly and i felt the heaviness of my dream body (like i would feel if i would try to fly IRL). I’ll try to free my mind more next time
I’ve another hypothesis.
Could it be possible that LDers never try to open doors because they are persuaded that they are in their “real” home, and that they couldn’t open “real” doors with their “ghost” hands ?
I used to open doors all the time. The best I could do was to make them be unlocked.
But that was awhile ago. I simply find walking through doors and flying through windows to be easier than opening them first.
Going through solid objects in a dream feels very uncomfortable to me. I actually feel wood, metal etc. scrape against my skin. It’s easier to turn the back to the door and order it to be gone when I turn around again. 
Maybe it is easier to remember going through a door or window than opening it in the normal way? We open doors all the time, every day. I can’t remember every door I have opened. We don’t break the laws of physics very often IRL. Doing something extraordinary like flying out through a window might stick in the memory easier.